First of all it should be noted that the structural intervention must guarantee some adequate safety levels depending on the "level" of intervention. In this case, the Italian legislation has always provided for two levels of intervention: seismic improvement and seismic adjustment. In the new version of the Technical Standards for Construction (Ministerial Decree of 17/01/18), a third level of local repair or intervention was also included.
The technical standards of construction frame the classification of interventions in seismic areas and identify the following categories of intervention:
- ADJUSTMENT INTERVENTIONS aimed at achieving the safety levels envisaged by these regulations;
- IMPROVEMENT INTERVENTIONS aimed at increasing existing structural safety, without necessarily reaching the levels required by these standards;
- REPAIRS OR LOCAL INTERVENTIONS are works that involve isolated elements, but that nevertheless lead to an improvement in the pre-existing safety conditions.
Adjustment and improvement interventions must be subjected to static testing.
The conditions that determine and oblige the seismic adjustment in accordance with the NTC 2018 are defined below:
- raising the building;
- expand the building through structurally connected works;
- make class and/or destination variations that lead to increases in global loads in the foundation of more than 10%; however, the obligation to proceed to the local verification of the individual parts and/or elements of the structure remains, even if they involve limited portions of the building;
- implement structural interventions aimed at transforming the building through a systematic set of works that lead to a building organization different from the previous one.
In any case of adjustement, the project must refer to the entire construction and must report the checks of the entire post-intervention structure, according to the indications of this chapter. A variation of the height of the building, for the construction of summit curbs, as long as the number of floors remains unchanged, is not considered superelevation or extension, according to points a) and b). In this case it is not necessary to proceed with the adjustment, unless the conditions referred to in points c) or d) above are met.
Having said that, in any case it is always advisable to make a curb at the top, also with alternative techniques to that of the more traditional concrete curb. For example, it is possible to use roofing solutions that use wooden tie-rods, provided they are properly designed and connected to the walls. In this regard, see also the contents of Annex 2 of the Ordinance P.C.M. 3431, and in the most recent Directive of the President of the Council for Cultural Heritage (DPC 12-10-2007) regarding coverage interventions:
«It is advisable, where possible, to use elements to strengthen the contact point between the masonry and the roof. In addition to the connection with metal hoists that prevent translation, we can make tie-rods made of wood or metal, appropriately connected both to the walls and to the wooden warping of the roof (metal caps), to form at the same time an upper edge of the walls to traction, an element of distribution of the loads to the supports of the roof warping and a constraint similar to a hinge between masonry and warping.»
The connection between the curb and the underlying masonry can be realized with different connection systems existing on the market (eg chemical plugs): the effectiveness of the connection does not depend only on the fixing system, but also and above all on the state of cohesion of the underlying masonry. In general, the use of curbs at the top, also made with other construction techniques, is recommended by the legislation both to improve the interaction with the roof and to increase the level of cohesion of the underlying masonry. Still in the same Annex 2 we can see:
«Curbs at the top of the wall can be an effective solution to connect the walls, in an area where the masonry is less cohesive due to the limited level of compression, and to improve the interaction with the roof; instead, the execution of curbs at the intermediate levels, performed in the thickness of the wall (especially of stone masonry) should be avoided, given the negative effects that the breached openings produce in the distribution of stresses on the walls. These can be realized in the following ways:
- in armed masonry, allowing the connection to be made using a technique aimed at the maximum conservation of existing wall features. In fact, they must be made with full-thickness masonry and good characteristics; generally the most natural solution is the use of solid brick masonry. Inside it must be housed a metal reinforcement, made adherent to the wall of the curb using conglomerate, for example cement mortar. The realization of connections between curb and masonry, carried out by means of reinforced piercings arranged with inclined course, if necessary it is effective only in the presence of good quality masonry. In other cases it is advisable to carry out a consolidation of the masonry in the upper part of the wall and rely on the adherence and contribution of friction.
- in steel, representing a valid alternative for their lightness and limited invasiveness. They can be executed through a light reticular structure, in angular elements and metal plates, or through plates or profiles on the two faces, connected to each other by means of passing bars; in both cases it is possible to realize an acceptable connection to the masonry without the need to resort to armed pierce. In the presence of poor quality masonry, the intervention must be accompanied by a work of reclamation of the band of masonry concerned. The metal curbs are particularly suitable for the connection of the wooden elements of the roof and contribute to the elimination of any thrusts.
- in reinforced concrete, only if of limited height, to avoid excessive weighting and stiffening, which have proved harmful because they produce high tangential stresses between curb and masonry, with consequent sliding and disintegration of the latter. In particular, these effects occurred in cases in which the roof structure had also been stiffened and weighed down. In the case of curb in c.a. it is generally advisable to consolidate the masonry near it, since the stiffness of the two elements is different in any case. The connection between curb and masonry can be improved by using perforated holes...»
